Encode the input and output parameters for a SimpleRequest instance in a
function type and move the CacheKind to the end, making it easier to
sort out inputs from outputs:
```
class DefaultTypeRequest
: public SimpleRequest<DefaultTypeRequest,
Type(KnownProtocolKind, const DeclContext *),
CacheKind::SeparatelyCached>
```
When building with Visual Studio, the following error is identified:
'SimpleRequest': non-class template has already been declared as a class template
note: see declaration of 'SimpleRequest'
Change the friend declarations to friend the type without the `class` specifier.
This patch removes the need for Request objects to provide a default
cycle-breaking value, instead opting to return llvm::Expected so clients
must handle a cycle failure explicitly.
Currently, all clients do the 'default' behavior, but this opens the
possibility for future requests to handle failures explicitly.
